Environmental stress cracking is the phenomenon whereby a stressed resin develops brittle cracks when exposed to a fluid such as a detergent or an organic liquid. This phenomenon can cause premature failure of articles manufactured from the resin. Environmental stress cracking resistance (“ESCR”) tests have been developed to measure the resistance of resins to their environment. One such test is described in ASTM D1693. ESCR is commercially important particularly when resins come into contact with detergents and organic chemicals, such as household chemical containers and organic chemical containers.
ESCR testing can also be used as a measure of a resin's resistance to slow crack propagation. Slow crack propagation occurs in resins that are at low stress levels, over extended periods of time. In this case a brittle crack propagates through the materials. This type of failure mechanism is seen in commercial applications of polyethylene in pressure pipe, containers, and vessels. Commercial polyethylene pressure pipe systems are designed to have a lifetime in excess of fifty years. Improved ESCR at high stiffness would be particularly desirable for such applications.
It is known in the art that lowering resin density of linear polyethylene resins, such as linear low density polyethylene (“LLDPE”), medium density polyethylene (“MDPE”) and high density polyethylene (“HDPE”), greatly improves the ESCR of the resins. However, this improvement in ESCR is at the expense of resin stiffness. As a result, conventional single reactor resins have a poor balance of ESCR and resin stiffness.
Resins with a bimodal molecular weight distribution, also termed “bimodal resins,” are resins having at least two polymer components with different average molecular weights. In this description, the resin with the higher average molecular weight is referred to as the “HMW polymer component”, and the resin with the lower average molecular weight is referred to as the “LMW polymer component”. Resins with a bimodal molecular weight distribution (“MWD”) can be produced in a single reactor using the technology disclosed in, for example, U.S. Pat. No. 5,539,076, or by the use of a series of reactors or reaction steps. For example, bimodal MWD polyethylene resins can be produced in a tandem slurry processes. Bimodal resins such as those produced in series reactors are known to have a good combination of high ESCR and stiffness, believed to be because the polymerization process is controlled to ensure that the comonomer is incorporated in the HMW polymer component. U.S. Pat. No. 4,461,783 to Baily et al. discloses that high ESCR, high density resins may be obtained with independently prepared, mechanically blended polyethylene resins of different MWD where the HMW polymer component contains the majority of the comonomer, and the LMW polymer component is essentially a homopolymer.
U.S. Pat. No. 5,539,076 to Nowlin et al. discloses the production of polyethylene resins with bimodal MWD in a single reactor using Ti/Zr bimetallic catalyst systems. However, in these resins, the comonomer is predominantly in the LMW polymer component of the bimodal resin. This type of comonomer distribution does not meet the requirements as disclosed in U.S. Pat. No. 4,461,783 for high ESCR at high resin density. Other background references include WO 00/50466, WO 98/57998, WO 99/31146, U.S. Pat. No. 5,624,877 to Bergmeister et al., EP 0 619 325 A1, and EP 0 882 744 A1.